Imaging devices, such as printers, facsimile machines, etc., often employ a print head for printing on a printable medium, such as paper. Ink is usually supplied to the print head from an ink reservoir via a flow passage. In one application, the ink reservoir and print head form a single unit, e.g., a print cartridge, and ink flows from the ink reservoir to the print head via the flow passage during printing. In another example, the ink reservoir and print head are separate, and during printing, ink flows from the ink reservoir to the print head via a flexible duct interconnecting the ink reservoir and the print head. Many print heads, such as used in ink-jet devices, include resistors that vaporize the ink supplied to the print head. This causes the ink to be ejected through orifices of the print head so as to print dots of ink on the printable medium.
To prevent ink leakage from the reservoir, it is common to exert a force on the ink to retain the ink within the ink reservoir. For example, many ink reservoirs contain a capillary medium, such as foam (or an ink sponge), that is capable of absorbing and retaining ink. The capillarity of the capillary medium exerts a force (capillary force) that draws the ink into the capillary medium, preventing the ink from leaking out of the capillary medium and thus the reservoir. Many ink reservoirs initially contain enough ink to wet the capillary medium up to a percentage of the height of the capillary medium above the bottom of the capillary medium, e.g., 75 to 95 percent, with the remaining upper portion of the capillary medium containing air, for example. Moreover, ink reservoirs often include an air-filled space between the top of the capillary medium and a cover of the ink reservoir.
Capillary medium-based ink reservoirs are typically vented to atmospheric pressure to prevent excessive vacuum pressures within the reservoir that can reduce or prevent ink flow to the print head, e.g., by a vent disposed in the cover of the ink reservoir. In this situation, air flows through the vent from an atmosphere surrounding an exterior of the ink reservoir to an interior of the ink reservoir. In addition, venting relieves pressure buildups that can occur when an ink reservoir is exposed to extreme environmental conditions, e.g., that can be encountered during shipping, such as high temperatures in motor vehicles or low pressures in airplanes at high altitudes. In this situation, air flows through the vent from the interior of ink reservoir to the atmosphere surrounding the exterior of the ink reservoir.
In some situations, air becomes trapped in the capillary medium, e.g., while adding ink to the ink reservoir, forming air pockets or voids within the capillary medium. This problem is amplified for applications involving hydrophilic capillary media because hydrophilic capillary media normally do not require a vacuum during filling. Moreover, when the ink reservoir is subjected to stresses, e.g. during shipping and/or handling, such as dropping the ink reservoir, the volume of entrapped air can increase or air from the space above the capillary medium can be displaced into the capillary medium. The air within the capillary medium causes problems when the ink reservoir is exposed to high temperatures and/or low pressures. In particular, the high temperatures and/or low pressures cause the air within the capillary medium to expand, forcing ink out of the vent instead of air.